Phonograph tone arms used in record players are generally of an elongated form and contain, at the one forward end, a pickup cartridge which includes a downwardly extending stylus thereon, the tip of which engages the grooves of the record for playing purposes. Intermediate of the forward and rearward ends of the tone arm, pivoting means are provided. Typically, a counterweight is employed on the rearward end of the tone arm, to provide for some adjustment of the pressure on the tip of the stylus by the weight and position of the counterweight.
The amount of stylus pressure typically measured in grams or a fraction of a gram or the stylus force is quite important. Excessive stylus force causes excessive record wear and may damage the record groove and affect playing performance, while insufficient force also affects record-playing performance and may permit the stylus tip to contact and damage the side walls of the record groove. In addition, since different pickup cartridges are used on tone arms with different weights, it is desirable to provide for a different or adjustable stylus force for optimum tracking. Thus, it is important to provide a stylus-force adjustment mechanism to be used in tone arm systems, and such mechanism should allow for the variation in weight of the pickup cartridges.
In practice, a number of adjustable counterweight systems have been proposed for use on phonograph tone arms. One system employs a sliding counterweight on the rearward end of the tone arm, which reads against stylus-force markings screened onto the exterior surface of the tone arm, whereby positioning of the slidable weight over the markings provides for desired stylus force.
Tone arms with counterweights are set forth, for example, in U.S. Pat. No. 3,093,379 which employs a weight which is adjusted by a threaded, precalibrated screw to the desired position. U.S. Pat. No. 3,167,317 discloses the use of a frictionally slidable counterweight and a threadably adjustable counterweight. U.S. Pat. No. 3,384,380 shows a counterweight secured to a screw for movement within a sleeve. U.S. Pat. No. 3,502,339 shows an adjustable counterweight with an adjustment of the weight by rotation of a screw. U.S. Pat. No. 3,417,999 provides a sleeve-like counterweight adjustable by a wheel on a sleeve. U.S. Pat. No. 3,416,807 shows a counterweight sleeve adjustable by pinion- or sleeve-engaging rack teeth on the tone arm. U.S. Pat. No. 3,378,267 discloses a spring-loaded sleeve counterweight on a tone arm.